1. Field of the Invention
The present invention relates to a process for removing impurities from the effluent produced during the washing of the hot gases generated during a sulfide concentrate roasting process, an apparatus for performing the process, and a plant utilizing the process. More particularly, the invention relates to a process of removing mercury and selenium impurities from a weak acid solution by precipitating them using sodium sulfide and sodium dithionite, respectively. The precipitates are then filtered from the effluent, which is then substantially free of impurities.
2. Description of the Related Art
During the roasting of sulfide concentrates, such as zinc or copper, the sulfur dioxide generated by heating the calcine is treated in an acid plant to be converted to the sulfuric acid end product. The sulphur dioxide gas produced in the roasting section is scrubbed to remove traces of volatile impurities such as selenium and mercury. These impurities are removed in an acidic solution, which may be reused for the scrubbing operation several times until the concentrations of selenium, mercury, and sulfuric acid in the solution reach levels too high for further recycling. Typically, at this point the solution is neutralized, the solids are removed and stored in a holding pond, and the effluent is returned to waterways. Due to increased environmental concerns, a need has developed for a process that will enable acid plants to remove additional selenium and mercury from the acidic solution before the neutralization step, thereby reducing the amounts of these impurities released into the environment.
It is known in the art to wash hot gases produced in various industrial processes to remove contaminants such as selenium and mercury. In U.S. Pat. No. 3,966,889, selenium is absorbed by an alkali metal sulfite or bisulfite from hot gas. The gas is then cooled and humidified, and is filtered through a glass fiber filter to remove any remaining selenium. The selenium may then be recovered from the absorbing solution by precipitating with SO.sub.2 as a reducing agent, and allowing the reaction mixture to stand for 3-5 hours at 50-90.degree. C. This is followed by another filtration step to remove the precipitate from the solution.
In U.S. Pat. No. 3,677,696 hot gases containing mercury are washed with sulfuric acid at 70-250.degree. C. Mercury and selenium may be found in the wash liquid, but no further process is described to remove these contaminants from the wash liquids.
In U.S. Pat. No. 4,579,726 hot gases are cooled and scrubbed with a sulfuric acid mist to separate mercury from the hot gases in the form of sulfatized mercury. The mist containing the sulfatized mercury is then separated from the hot gases, but no method for further removing the mercury from the mist is disclosed.
Methods of removing selenium and mercury from aqueous solutions produced in industrial processes are also known. U.S. Pat. No. 3,695,838 discloses a method of precipitating mercury from solution using H.sub.2 S gas, or in the alternative, by reduction with metals such as zinc, aluminum, and iron, or by reducing with a low molecular weight aldehyde.
In U.S. Pat. No. 3,577,216 selenium is recovered to be reused as a catalyst in organic oxidation reactions by using an oxide, hydroxide, or metal salt to precipitate the selenium from an acidic solution as a metallic selenite.
In U.S. Pat. No. 3,933,635 selenium is precipitated from an acidic solution using powdered zinc, aluminum or iron at a temperature of 25-85.degree. C., at pH 1-4, for 1-10 minutes.
In U.S. Pat. No. 3,959,097 trace amounts of selenium are precipitated from an acidic solution using a metal hydroxide in the presence of a sulfide at 85.degree. C. or greater, under a pressure of 160 psig or greater.
U.S. Pat. No. 5,200,082 teaches a process of removing selenium from an acidic solution by filtering the solution, heating it, adding powdered iron in the presence of elemental sulfur as a reducing agent, agitating the mixture, and oxidizing the solution with continued agitation. This results in precipitation of the selenium from the solution.
U.S. Pat. No. 4,330,508 discloses a process of removing selenium from an acidic copper sulfate solution by passing the solution through a tubular member in a plug flow manner, and injecting SO.sub.2 or a sulfite solution (which may be sodium sulfite or sodium hydrogen sulfite). This process normally does not allow enough selenium to be removed to produce copper by electrowinning that is free of selenium contamination. The '508 patent also discloses that selenium may be precipitated using sodium sulfite as a reductant at atmospheric pressure.
U.S. Pat. No. 4,377,556 teaches removal of selenium from acidic copper sulfate solution by reacting the solution with a stoichiometric excess of SO.sub.2 or sulfite at a temperature of at least 140.degree. C., and then passing oxygen into the solution to oxidize all dissolved cuprous ions to cupric ions, reducing the likelihood that copper will precipitate from solution when it is cooled. The selenium reducing sulfite may include bisulfite and pyrosulfite ions.
The above-described methods have several disadvantages, including the use of costly powdered metals as reducing agents, difficulty in reducing the amount of selenium and mercury in solution to acceptably low levels, and the expense of providing equipment capable of carrying out the reactions, particularly where the reactions require the use of high temperatures and pressures. These methods can also present significant hazards due to the possibility of igniting volatile selenium hydride and arsenic hydride gases.
It is desirable to develop a process for removing selenium and mercury from solutions produced during the roasting of sulfide concentrates to produce sulfuric acid, or during any other industrial process, that is easy to perform, cost-effective, and prevents further environmental damage due to contamination of waterways with selenium and mercury. None of the above-mentioned techniques addresses a method of removing mercury and selenium from acidic solutions using sodium sulfide to remove the mercury, or sodium dithionite (also known as sodium hydrosulphite) to remove selenium, as are disclosed in the present invention.